Morphing flames and localized hot spots: Unlocking the dynamics of deflagration-to-detonation transition in curved channels

Abstract

This study presents a systematic experimental investigation into the flame propagation and deflagration-to-detonation transition (DDT) processes within curved channels, with a particular focus on the influence of initial pressure and geometric parameters (inner and outer radii). The experimental setup, featuring a 270° curved channel and utilizing stoichiometric ethylene-oxygen mixtures as the fuel, employed high-speed camera to capture flame dynamics and pressure transducers to monitor pressure wave distributions. The results demonstrate that the geometric characteristics of the curved channel significantly modulate flame acceleration, with smaller outer radii and larger inner radii markedly enhancing flame acceleration and reducing the time required for DDT (t_DDT). The initial pressure emerges as a key parameter governing the spatial distribution of hot spots and the onset of detonation. Higher initial pressures drive hot spots toward the flame front, accelerate energy accumulation, and significantly improve DDT efficiency. The experimental findings demonstrate that hot spots exclusively form near the outer wall of the channel, a phenomenon attributed to localized high-temperature and high-pressure regions induced by the interaction between the leading shock wave and the outer wall. Under varying initial pressures, hot spots exhibit three distinct spatial distribution patterns: (1) at the root of the tongue-shaped flame, (2) at the tip of the tongue-shaped flame, and (3) simultaneously at both the root and tip. Notably, the presence of flame front wrinkles at the tip of the tongue-shaped flame is identified as a key feature in the latter two patterns, playing a dominant role in hot spot formation. Quantitative analysis further reveals a positive correlation between the characteristic length of the tongue-shaped flame at the moment of the hot spot formation and t_DDT. This finding highlights the synergistic interplay between the geometric properties of the curved channel and initial conditions in determining the efficiency and characteristics of the DDT process. Collectively, this study provides critical experimental insights and theoretical support for understanding the complex physical mechanisms underlying flame propagation and detonation transition in curved channels, offering valuable implications for optimizing combustion dynamics in geometrically complex systems.